<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
    #box1 {
      width: 400px;
      height: 400px;
      border: 1px solid purple;
      position: absolute;
    }

    #box1 button {
      width: 100px;
      height: 40px;
      position: absolute;
      left: 50%;
      bottom: 10px;
      transform: translateX(-50%);
    }
  </style>

</head>
<body>

<div id="box1">

  <!--
    将按钮定位到父div的下底边正中间偏上的位置
  -->
  <button>刷新</button>
</div>

</body>
</html>
